In cases when a very reliable supply of electrical power is needed and/or when available electrical power sources are unreliable, it may be necessary to be able to switch between different AC power sources to prevent or at least reduce loss of the supply of electrical power to a power consumer.
To provide such a switch between different AC power sources, it is known to use electromechanical transfer switches which connect or disconnect AC power sources by physically closing or opening electrical circuits.
Such conventional electromechanical transfer switches have various drawbacks. For example, if a first AC power source fails, there may be a period of interruption of supply of electrical power until a second AC power source is connected. Furthermore, the electromechanical switching may lead to latching and high current transients.
U.S. Pat. No. 5,138,184 discloses a so-called static transfer switching apparatus for switching between AC power sources. This type of apparatus can switch faster than the above-mentioned conventional electromechanical transfer switches.
The static transfer switching apparatus according to U.S. Pat. No. 5,138,184 may be well suited for feeding AC loads. When it is desired to feed DC loads, such as batteries etc, the output from the static transfer switching apparatus needs to be rectified using additional circuitry, such as full wave rectifier bridges. This results in power losses and thermal stress, which may be particularly detrimental in renewable energy applications with high efficiency requirements or in applications where high ambient operating temperatures and limited cooling possibilities is a significant concern.